What does horizontal gaze nystagmus refer to?

Horizontal gaze nystagmus refers to an involuntary jerking of the eyes that occurs when a person is asked to follow a moving object with their gaze. This phenomenon is often tested during field sobriety evaluations to assess a person's level of impairment, particularly in cases of suspected driving under the influence of alcohol. When a person is under the influence, their eye movement can become affected due to the depressant effects of alcohol on the central nervous system, leading to noticeable jerking movements of the eyes as they attempt to follow a stimulus. This involuntary eye movement can be an important indicator of impairment and serves as one of several assessments used by law enforcement to determine a person's ability to operate a vehicle safely.